(a) Field of the Invention
The present invention relates to a display device. More specifically, the present invention relates to an organic electroluminescent (EL) display device with an improved aperture ratio.
(b) Description of the Related Art
The organic EL display device, which is a display device for electrically exciting a fluorescent organic compound to emit light, has organic light-emitting cells that are voltage- or current-driven to display an image. These organic, light-emitting cells have a structure composed of an anode layer, an organic thin film, and a cathode layer. To balance the electrons and holes in order to enhance luminescent efficiency, the organic thin film has a multi-layer structure that includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The multi-layer structure of the organic thin film can also include an electron injecting layer (EIL) and a hole injecting layer (HIL).
As shown in FIG. 1, an organic EL display device includes an organic EL display panel (referred to as “display panel” hereinafter) 100, a data driver 200, and a scan driver 300.
The display panel 100 includes a plurality of data lines D1 to Dm arranged in the column direction, a plurality of scan lines S1 to Sn arranged in the row direction, and a plurality of pixel circuits.
Each of the pixel circuits includes a driving transistor 20 for controlling the current flowing to an organic EL element 40, a switching transistor 10 for applying a voltage at the data line D1 to a gate of the driving transistor 20 in response to a select signal provided by the scan line S1, and a capacitor 30 coupled between the gate and the source of the driving transistor. The drain of the driving transistor 20 is coupled to a power source line 50 for transmitting a power source voltage VDD.
The data driver 200 supplies data voltages to the data lines D1 to Dm, and the scan driver 300 sequentially applies select signals for selecting pixel circuits to the scan lines S1 to Sn.
FIG. 2 shows a plan view of a pixel circuit coupled to the scan line S1 and the data line D1 in the organic EL display device shown in FIG. 1, and FIG. 3 shows a cross-sectional view of the part of A-A′ of FIG. 2.
As shown in FIGS. 2 and 3, a gate electrode 16 of the switching transistor 10 is formed on the same electrode layer as that of the scan line S1, and a source region 13 of the switching transistor 10 is coupled to the data line D1 by a contact hole. Drain region 14 of the switching transistor 10 is coupled to a gate electrode of the driving transistor 20 through a contact hole. The drain region of the driving transistor 20 is coupled to the power source line 50 through a contact hole, and a source region is coupled to the pixel electrode layer 42 of the organic EL element 40 by a contact.
Transparent insulation film 12 is formed on a substrate film 11. A first insulation film 15 is formed on the polycrystalline silicon layer, and a gate electrode 16 is formed to cross the polycrystalline silicon layer on the first to insulation film 15.
Part of the polycrystalline silicon layer beneath the gate electrode 16 is not doped, and two parts thereof are doped with n-type dopant. The regions doped with the dopant form a source region 13 and a drain region 14 respectively, and the undoped region forms a channel region.
A source electrode 18 is formed on the source region 13, and the source region 13 is coupled to the data line D1 through the source electrode 18. A drain electrode 19 is formed on a drain region 14, and the drain electrode 19 is coupled to a gate electrode of the second transistor 20.
The organic EL element 40 comprises an organic EML 41 and a pixel electrode layer 42, such as indium tin oxide (ITO). The organic EL element 40 is positionally separated from the power source line 50. A cathode electrode 21 is formed on the organic EML 41.
The organic EML 41 is formed at a pixel region defined by an insulation film which forms an aperture on the pixel electrode layer 42. That is, since the organic EML 41 is formed within the pixel electrode layer 42, the region for forming the organic EML 41 is limited by the pixel electrode layer 42. Therefore, the narrow region of the generated organic EML 41 degrades the aperture ratio of the pixel circuit. It is therefore desirable to improve the aperture ratio of an organic EL display device.